1. Field of Invention:
This invention relates to measuring the various liquid levels in a holding tank which is vented to a vacuum (pressures less than ambient air pressure surrounding a tank). Particularly it uses a bubbler tube open to the bottom of the holding tank and bubbles air up through the liquid in the tank. An example of this invention would be the measurement of the sewage level in the waste tank of a commercial jet aircraft.
2. Description of Prior Art:
Modern aircraft are equipped with toilets for use by the passengers and the crew. These toilets usually (2 or 3) flush into a single sewage waste tank. The waste tank is vented to the partial vacuum (low atmospheric pressure) existing at flying altitudes, or as created by a vacuum pump, if the aircraft is on the ground. When toilets are flushed into the waste tank there is a driving rush of air from the compressed cabin atmosphere as it escapes down the toilet toward the waste tank, pushing all the sewage ahead of it. As a result the sewage is violently foamed and splattered over the inside of the waste tank as it enters. This splattering is also caused in large part by the entrained gasses and air held in the waste at cabin air pressure, but becomes explosive when that pressure is released in the partial vacuum of the waste tank. In the past, all prior art which has been installed internal to the holding tank has been coated and caked with dried waste rendering them unreliable and inaccurate. Several methods of continuous sewage measurement have been tested with varying degrees of success. One of the most promising methods which is being developed is measuring the differential pressure between the top and the bottom of the holding tank. However this last measuring method is both expensive and heavy, and uses sewage operating against a sensitive pressure diaphragm as the medium of measurement. The contact with sewage against this bottom sensitive diaphragm is troublesome when subjected to turbulence, contamination, corrosion and lower sewage line freezing.
The present invention overcomes these problems of the prior art by having the sensitive pressure diaphragm external and remote from the waste tank while using clean air as the interface between the sensitive diaphragm and the sewage. Thus the present invention is free of the operating drawbacks of the prior art. The present invention is able to give reliable and accurate readouts of the liquid level in the tank and it is conveniently sized and inexpensive to produce.
For a better understanding of this invention the following symbols and definitions relate to the sign convention of air pressure as referenced in this patent application:
P1=refers to aircraft passenger cabin air pressure exterior to a waste tank (see FIG. 1) This pressure, P1, as referred to in this application is a gauge pressure reference point. Therefore a standard pressure gauge mounted external to a tank with its pressure port disconnected would read 0 PSIG(=P1). This reference point will normally fluctuate and be in the range of 7.0 to 14.7 PSI absolute. Therefore negative pressures as referenced in this application are less than P1. The reason the P1 reference point changes is whether an aircraft is on the ground or flying. Cabin air pressure pumps pressurize the cabin at approximately the 8,000 foot level of altitude even though the aircraft may fly at much higher altitudes where lower pressures exist.
P2=refers to the pressure inside a bubbler tube and is normally less than P1. This negative air pressure or partial vacuum is found at a point in this sensor known as a bubble source point when the sensor system is stable and operating. Due to the low air flow rate through the bubbler system and the large size of the air flow passages, P2 at the bubble source point is negligibly different from the P2 pressure acting on the differential pressure diaphragm.
P3=refers to the pressure above the sewage in the waste tank and is normally less than P2 when the sensor is running. Thus P3 is a negative pressure in the top of the waste tank above the liquid and in a vent line above the waste tank. If an aircraft is operating at a flying altitude, P3 is maintained by the negative air pressure found out side the aircraft. But if an aircraft is on the ground, P3 is maintained by vacuum pumps.
P4=refers to the air pressure found out side an aircraft. If the aircraft is on the ground P4 is the same as P1, but if the aircraft is flying it is that negative atmospheric pressure or partial vacuum found at that flying altitude.
When I use the term "vacuum" in this application I mean any negative air pressure which is less then P1.